1. Field of the Invention
The present invention relates to a method of processing a signal from a thermal type flow sensor for detecting fluid flow, and more particularly, to a method of processing the signal to improve response characteristic when the flow is changed.
2. Description of Related Art
Heretofore, for a thermal type flow sensor, there has been well known a thermal type air flow sensor for detecting intake air flow in an internal combustion engine, and for a typical example of that type, there is well known a hot wire type air flow sensor which controls an electric current to be sent to a platinum wire so as to fix the temperature of the platinum wire in an air intake passage and which receives an air flow signal in accordance with the value of the electric current. In addition, for a more inexpensive hot wire type air flow sensor, a thermal type air flow sensor, in which platinum is deposited on an alumina substance or film as a replacement for the platinum wire, has been eliciting considerable interest.
In those thermal type air flow sensors, since air flow is measured by detecting the electric current sent to a temperature dependent resistance and controlled to fix the temperature of the resistance, when air flow to be measured is changed by the resistance of the temperature dependent resistance and by the thermal conduction and heat reserve of the resistance's holding member, it is well known that output response is so delayed as to produce error in its detection characteristic. For example, in a hot wire type air flow sensor in which a platinum wire is nearly stretched in an air intake passage, there is a comparatively little effect caused by the holding member; however, both in a hot wire type air flow sensor in which a platinum wire is coiled up around a ceramic bobbin and also in the thermal type air flow sensor in which platinum is deposited on an alumina substrate or film as described above, there is considerably great effect of the above-noted heat conduction and heat reserve upon the ceramics or the film which are holding member.
FIG. 1 illustrates this type of thermal type air flow sensor, in which reference numeral 11 designates a housing in the form of a pipe through which air flows, the direction of flowing air being shown by the arrow. Reference character R.sub.H designating the temperature dependent resistance for detecting air flow, is formed by printing or depositing platinum in a meander form onto an alumina substrate 14 as shown in FIG. 2, is then trimmed, and is arranged in the air passage with other resistances R.sub.K and R.sub.M. Resistances R.sub.H, R.sub.M, R.sub.K, R.sub.1, and R.sub.2 are arranged in a well-known bridge circuit to form a resistance value detecting unit, and those resistances form a closed loop with a differential amplifier 12 so that R.sub.H can be controlled to have a fixed temperature or fixed resistance value. Consequently, the electric current sent to R.sub.H is determined according to the air flow and an output voltage 13 can be obtained according to the product of the value of the electric current and the resistance value of R.sub.M.
Next will be described below the response delay which occurs in such a thermal type air flow sensor 1 when air flow is changed. FIG. 3 is an illustration of the response in the thermal type air flow sensor when air flow is changed gradually, and the response characteristic is shown substantially equal to the line having a bending point shown as A. In the figure, the ordinate designates the elapsed time after the change of air flow in the staircase, and the abscissa designates the change rate of air flow. Time lag to the point A is caused mainly by the delay of the thermal response of the platinum resistance R.sub.H and the response of the circuit, and the deviation of the value at the point A from a desired value and the time until the value is converged from the point A to the desired value are mainly caused by heat conduction and heat reserve of the alumina substrate 14 which is a holding member for the platinum resistance R.sub.H. FIG. 4 is an illustration to show the above-mentioned operation, showing temperature distribution of the alumina substrate 14 with the horizontal axis designating distance which is based upon the position of the platinum resistance R.sub.H to the alumina substrate 14. The temperature around the platinum resistance R.sub.H is controlled to be fixed in a temperature high enough compared with the air temperature by the aforementioned circuit. Then, heat being generated in the platinum resistance R.sub.H is emitted into the air, while the heat is transmitted and reserved into the alumina substrate 14 from the platinum resistance R.sub.H. In order to compensate for this heat loss, the closed loop controls the electric current to the platinum resistance R.sub.H. Then, the output of the thermal type air flow sensor 1 to a predetermined air flow results in including the heat conduction and heat reserve to the alumina substrate 14, however, there can be obtained a characteristic under a state that heat of the alumina substrate 14 is in equilibrium, that is, an accurate flow characteristic under a stationary state. On the other hand, where air flow is changed, such thermal equilibrium as aforementioned can not be obtained here, then, error in the flow characteristic will be generated. In FIG. 4, a line l1 designates the temperature distribution when there is small air flow, so there is a line l2 when there is large air flow. In the figure, the line l2 is shown in the position below the line l1 because the cooling effect of the alumina substrate 14 depends on flowing air flow. When air flow is gradually changed from small to large, the temperature distribution is finally to be the line l2, however, at an initial stage, the air flow is corresponding to the line l2 but has the temperature distribution of the line l1, and a supply current to the platinum resistance R.sub.H, that is, output of the thermal type air flow sensor 1 becomes smaller than the substantial output thereof. To be concrete, when any change in air flow is occurs, there is generated initial flow error corresponding to the difference between the temperature distribution corresponding to the air flow before the change and the temperature distribution corresponding to the air flow after the change, and the time error until the temperature distribution becomes stationary to the air flow after the change will be maintained while gradually decreasing. The degree of the initial flow error and time error, in such a thermal type air flow sensor 1 as shown in FIG. 1, depends greatly on heat conduction and heat reserve of the alumina substrate 14 which is a holding member. And even in a fuel injection system of an internal combustion engine which is manufactured in consideration of the acreage of platinum resistance R.sub.H and the thickness of the alumina substrate 14 and the like so as to have responsiveness and durability to be practically usable, there is still the maximum 30 percent of the initial flow error and about 500 msec of the time error continuing, and it is substantially difficult to permit this response delay in controlling fuel of the internal combustion engine.
In order to improve such disadvantage as described above, as shown in Japanese Patent Application Laid-Open No. 63-134919, for example, a method of improving the response characteristic to the air flow change in the thermal type air flow sensor by devising its constitution is well known. The thermal type air flow sensor of prior art as described above, however, has problems in that its constitution is too complicated to be easily manufactured and its costs are prohibitive.